This invention relates to an optical information recording medium of write once (through a heat mode) type such as DVD-R, which is capable of performing a recording or reproducing data by means of a laser beam of a shorter wavelength.
The CD-R is well known as a means for recording and reproducing data such as images of character and graphic or music. The CD-R now available is formed by making use of a recording material, which is capable of recording and reproducing with a laser beam of 770 to 830 nm in wavelength and selected based on its absorbency and optical parameters (in particular, refractive index "n" and extinction coefficient "k"). For example, an optical disk comprising a dye layer containing pentamethine-based cyanine dye is employed.
Recently however, DVD-R (a digital video disk or a digital versatile disk), which is capable of recording and reproducing in high density with a red laser beam of 620 to 690 nm in wavelength for instance, which is shorter than the laser beam employed in the aforementioned CD-R, is now propagated as new media of the next generation.
Since the wavelength zone for performing the recording and reproducing in the DVD-R differs from that of the CD-R, the criteria for selecting the recording material to be employed for the CD-R cannot be applied to the DVD-R, and hence new selection criteria are required to be established from a viewpoint peculiar to the DVD-R.
Since the recording and reproducing of data in the CD-R as a write once type optical information recording medium are performed by making use of a semiconductor laser having a wavelength of 770 to 830 nm (.lambda..sub.1), it is considered that a recording layer comprising a dye layer enabling an additional recording should preferably be formed on a transparent substrate in such a manner that the optical parameters in particular, the refractive index "n" and extinction coefficient "k" of the recording layer at the aforementioned .lambda..sub.1 fall in the ranges of: 1.6 &lt;n&lt;4.0 and 0.01 &lt;k&lt;0.2 for the purpose of ensuring a suitable reflectance (65% or more) before recording and a modulated amplitude after recording. Namely, if refractive index; n falls outside the aforementioned ranges in the CD-R, it is impossible to assure a reflectance of 65% or more in the recording/reproducing wavelength zone thereof. On the other hand, if extinction coefficient; k falls outside the aforementioned ranges in the DVD-R, it is not only impossible to assure a sufficient reflectance but also impossible to perform a high quality recording. It is generally considered that, in order to assure the aforementioned preferable ranges of these optical parameters n and k, the maximum absorption wavelength .lambda..sub.2 of the recording layer should preferably be confined to the range of 600 to 750 nm. However, since the recording and reproducing are performed at a wavelength of 620 to 690 nm (.lambda..sub.2) in the DVD-R, i.e. which is far shorter than 770 nm, the aforementioned ranges of n and k, which are preferable for CD-R are no more preferable for DVD-R. Therefore, it is impossible for the DVD-R to suitably perform the recording and reproducing by making use of the recording materials which have been employed for the CD-R.